High performance color television monitors are used at television stations and in studios in place of conventional television receivers in order to provide a superior television image with a high degree of accuracy in such factors as color saturation, hue and white balance. In order to attain this high accuracy, these factors are not preset by hardwired circuitry, but rather are adjustable and may be brought into exact accordance with standard or selected values for these factors.
In the prior art, it was common for these factors to be adjusted by a highly experienced operator who manually adjusted various controls while watching a display of color bars on the display screen of the cathode ray tube (CRT) of the monitor. When color saturation and hue are to be adjusted, the display screen displays standard color bars from a color bar generator representing the standard values of the individual factors together with color bars generated by the monitor and the operator makes adjustments to match the colors. Color balance adjustment similarly requires a matching of light intensities for the three primary color signals. However, this manual adjustment requires a highly skilled, experienced operator with an excellent eye for colors. When only less able operators are available, the matching process is imprecise and unsatisfactory.
Therefore, a color television monitor system in which these factors may be adjusted automatically has been proposed in U.S. patent application Ser. No. 06/849,528, filed April 8, 1986 and assigned in common with the present application. In this system, when the color saturation and hue are to be adjusted, the monitor is supplied with a color bar signal from a color bar generator. The color bar generator generates an output signal in which each horizontal line period is divided into successive periods during which signals corresponding to respective predetermined colors are generated, so that the television image is constant and appears as a series of vertical stripes or bars of those colors, respectively. The monitor demodulates three primary color signals from the selected color bar signal by extracting the red signal R during that portion of the horizontal period when the red bar is generated, the blue signal B during that portion when the blue bar is generated, and the green signal G during that portion when the green bar is generated. In the present specification, the portion of the horizontal period during which each color bar is generated will be referred to as the period of the respective color bar.
The color saturation and hue can be adjusted by sampling the signals produced when different ones of the color bars are being generated. Specifically, if the color saturation is correctly adjusted, the level of the blue signal B obtained when the white bar is generated will be equal to the level of the blue signal B obtained when the blue bar is generated. Correspondingly, if the hue is correctly adjusted, the levels of the blue signal B obtained during the periods of the white bar, magenta bar and cyan bar should all be the same.
Therefore, in order to adjust the color saturation to the correct level, the color gain of the color television signal is adjusted so as to equalize the levels of the blue signal B during the white bar period and the blue bar period. Correspondingly, in order to correctly adjust the hue, the phase of the color subcarrier signal is controlled so as to equalize the levels of the blue signal B during the white bar period, the magenta bar period and the cyan bar period.
In addition to these adjustments, the system proposed in the above-identified application permits an automatic adjustment of the white balance. The color temperature defines the shade of white generated by a color picture tube. A black object turns different colors as it is heated and at a temperature of 6800.degree. K. it turns to a shade of white that corresponds to a standard white raster in the NTSC system. Proper adjustment of the color temperature for neutral white balance results in pictures of sharp contrast. In the color television or monitor, the sensation of white light is produced by combining equal energy, red, green and blue light. Color temperature adjustment sets the beam currents so that a white light output is obtained. If all the color phosphors on the screen of the CRT were of equal efficiency, equal beam currents would produce a neutral white raster, but in practice the color phosphors differ in efficiency. Therefore, white balalnce is adjusted by looking at or sensing the colors or intensities of the red, green and blue signals and adjusting them to produce a good white raster.
To achieve the automatic adjustment in the proposed system, a uniform, low brightness red image is displayed on the entire screen of the CRT and the brightness thereof is measured by a sensor. The low brightness red signal has a signal level of between ten and twenty IRE. The measured value of the brightness is compared to a reference color temperature value and the gain and bias of the driving circuit of the CRT are controlled to change the level of the red signal until the measured value equals the reference value.
Next, a high brightness red image is displayed on the screen of the CRT and the brightness thereof is measured by the sensor. The high brightness red signal has a signal level of 100 IRE. Again the gain and bias of the driving circuit are adjusted to make the measured value equal to a high brightness red reference value. Thereafter, low brightness and high brightness green images and low brightness and high brightness blue images are displayed on the screen and measured, and again the gain and bias of the driving circuit are adjusted to make the measured values equal to the reference value.
However, when the white balance is adjusted in this manner, ambient light from lamps, lighting fixtures, etc. will add to the light emitted from the screen of the CRT and will be included in the total brightness measured by the sensor. Consequently, the measured data indicates a brightness greater than the actual brightness produced by the CRT alone, and under such circumstances there will be an error in the white balance adjustment. In particular, when the low brightness images are measured and adjusted, the ambient light is comparatively large as compared to the brightness from the screen, and so the data produced by the sensor reflects the ambient light to a comparatively large degree. When the high brightness colors are measured and adjusted, the ambient light is relatively small. Thus, there will not only be an absolute difference between the light data measured and the light produced by the screen, but the proportional error produced by the ambient light will differ from the low brightness images to the high brightness images.
In an effort to avoid these difficulties, the sensor is arranged within a suction cup which may be removably attached to the screen of the CRT to cut off the ambient light as much as possible. However, this arrangement is not completely satisfactory because some ambient light may still reach the sensor by passing through the glass screen and being reflected into the sensor. Given the extremely high degree of accuracy required for this type of monitor, even this degree of error is undesirable.
There is a second difficulty associated with the sensor. Typically, the sensor is formed of a photovoltaic photodiode and the output current thereof is typically very small, for example 0.55 A at 100 lux. Consequently, it is necessary to greatly amplify the output current from the sensor, with a high gain amplifier, and it is well known that in such high gain amplifiers the offset and drift can be significant. While a chopper amplifier may be used to eliminate the offset and drift, such amplifiers are generally complicated and/or expensive. Consequently, the response of the high gain amplifier to the output current from the sensor will vary over time, presenting a highly disadvantageous feature in such an exacting adjustment as the while balance adjustment in a high performance color television monitor.